I have a 69 XR7 which has the low fuel light. When I first got the car, it looked like it was really empty and when I put in 5 gallons, the needle didn’t move much. I had already checked the gauge using resistors so I know that it’s reading correctly as long as it gets the right signal. Yesterday I was at a quarter tank so I stopped to add some fuel and it clicked off just short of 8 gallons because the tank was almost full. When I got underway, the gauge was reading 3/4.
I started to look online for replacement gauge that included a low fuel light and noted at West Coast is that the replacement gauge will only read 3/4 on the tank is full, which is why they recommend you send in your original sending in it for a rebuild, which is not an option for me. I also saw several post that said if you bend the arm that the float sits on that you can get the gauge to read correctly. I have a couple of questions: #1, does that work, and how much do you need to bend the arm? #2, if it does why don’t the suppliers provide descending unit With the arm at the right angle to begin with?
All very good questions.
I’m the guy that rebuilds these. Never bend the float arm. Guys think it works like a float in a toilet tank and it definitely does not. There are built in limits for the motion of the float arm. If you bend it down the sender will hit the limit sooner but still not read correctly. All calibration happens inside the resistor housing. These are tiny adjustments and only done after all of the connections have been zero’ed out. ( series resistance reduced to as close to zero as possible)
The reproduction senders do not duplicate the non linear resistor Ford used in these senders. That means the best you can hope for is a correct reading at full and empty if it was correctly calibrated. They also don’t really calibrate them. Because the resistance value is so far off in the middle the gauge reading will generally read too low way to fast.
Rebuilding your OEM sender costs $125 including a new brass float, thermistor that actually works, filter, seal ring and return postage in the USA.
So to specifically answer your questions: Bending the float arm makes it worse. Those people are guessing not knowing how it actually works. The reproduction senders generally do have the correct geometry. However some retailers claim that one size fits all, and that is absolutely not true. The senders for every year Cougar from 67 to 71 are different. The sender must match the size of the tank. Because of one size fits nothing some people do try to bend the float arm to try to make them work, and it can help a little but for all the reasons above it is a waste of time.
I will say one more time DO NOT bend the float arm. If you do manage to force the internal sweep arm to far you will short out the gauge and damage it, or the instrument voltage regulator or both. It is never a good idea to create a short circuit inside a gas thank. Most of the senders I can’t rebuild were damaged by owners trying to DIY the repair. I have written extensively about what is involved so please read up before you attmept to DIY. Finally, you will need to have excellent soldering skills to do this without melting important parts, and no I don’t sell the thermistor it is easily damaged by heat and you have to solder it in place.
Hi. Thank you for your reply.
I don’t have the OEM sender, so unless I run across one or spend a lot on one of the no core rebuilt ones I’m kind of struck. I just acquired the car last fall and it has what appears to be a new tank and sender. Based on the behavior I’m guessing it’s the one with the Crappy rating on your site.
When full it reads 3/4, which from what I’ve read corresponds to about 16 ohms. Full is 8-12 ohms. I tested the gauge some months ago when it was reading empty all the time by putting a 10 ohm resistor between the plug and ground, and the gauge read full. I also put a 73 ohm resistor in line and the gauge read E (just E, not below it).
I wish I knew what the “crappy” one reads when empty. If I knew then I could use a resistor in parallel to adjust the resistance the gauge sees to be 70, so that at least I would really know when empty is coming, that’s really what’s important. It would be a pretty big resistor, probably 100 ohms, so there’s no chance I would push the FULL value below 10 ohms and risk damage to the gauge.
(If it’s at 16 now when the tank is full then 1/16 + 1/100 = 1/13.8. At 13.8 ohms it will be closer to F but not there, and still above 10)
Will, I sent you a PM. Maybe I can help.